Previous studies have shown that ADP-actin undergoes an equilibrium polymerization with a critical concentration of 8 MuM (in l mM MgCl-2 and 0.2 mM ADP) while ATP-actin is a steady state polymer with a critical concentration of 0.35 MuM (in l mM MgCl-2 and 0.2 mM ATP). The hydrolysis of ATP that accompanies polymerization of ATP-actin occurs on the filament subsequent to, and slower than, the polymerization step so that the actin filament in ATP has an ATP-cap at one or both ends. It is this ATP-cap that stabilizes the filament, which consists mostly of ADP-actin subunits. We have now found that an equilibrium, all ATP-actin polymer can be transiently formed when high concentrations of actin are polymerized under continuous sonication. This transient polymer has a critical concentration of 3 MuM, suggesting that the steady state polymer is stabilized by the heterologous interaction of ATP-actin and ADP-actin subunits at the interface between a short ATP-cap and the ADP-core. This was confirmed by showing that the rate of nucleation and elongation of mixtures of ATP-actin and ADP-actin were faster than the rates with either species alone. These, and other, studies have led to a new model for actin polymerization which accurately predicts the experimental polymerization curves. In addition, experiments were carried out with the barbed filament ends blocked with either gelsolin or cytochalasin. In this way, it was found that only the barbed end, and not the pointed end, has an ATP-cap at steady state while both ends may be capped at monomer concentrations greater than the critical concentration. These experiments also allowed calculation of the association and dissociation rate constants and the critical concentrations for both the barbed and pointed ends of the equilibrium ADP-actin polymer and the steady-state ATP-actin polymer.